1. Field of the Invention
The present invention relates to a joint device for capacitive microwave coupling, and particularly to a joint device for capacitive microwave coupling which is suitable for coupling of a superconducting device with room temperature circuitry.
2. Description of Related Art
Electromagnetic waves called "microwaves" or "millimetric waves" having a wavelength ranging from tens of centimeters to millimeters can be theoretically said to be merely a part of an electromagnetic wave spectrum, but in many cases, have been considered from an engineering view point as being a special independent field of the electromagnetic waves, since special and unique methods and devices have been developed for handling these electromagnetic waves.
In the case of propagating an electromagnetic wave in frequency bands which are called the microwave and the millimetric wave, a twinlead type feeder used in a relative low frequency band has an extremely large transmission loss. In addition, if an inter-conductor distance approaches a wavelength, a slight bend of the transmission line and a slight mismatch in a connection portion causes reflection and radiation, and propagation of the electromagnetic wave is easily influenced from adjacent objects because of electromagnetic interference. Thus, a tubular waveguide having a sectional size comparable to the wavelength has been traditionally used. The waveguide and a circuit connected to the waveguide constitute a three-dimensional circuit, which is larger than components used in ordinary electric and electronic circuits. Therefore, application of the microwave circuit has been limited to special fields.
However, miniaturized devices composed of semiconductor have been developed as an active element operating in a microwave band. In addition, with advancement of integrated circuit technology, a so-called microstrip line having a extremely small inter-conductor distance has traditionally been used instead of tubular wave guides.
In general, the microstrip line has an attenuation coefficient that is attributable to a resistance component of the conductor. This attenuation coefficient attributable to the resistance component increases in proportion to a root of a frequency. On the other hand, the dielectric loss increases in proportion to increase of the frequency. However, the loss in a recent microstrip line is almost attributable to the resistance of the conductor in a frequency region not greater than 10 GHz, since the dielectric materials have been improved. Therefore, if the resistance of the conductor in the strip line can be reduced, it is possible to greatly elevate the performance of the microstrip line. Namely, by using a superconducting microstrip line, the loss can be significantly decreased and microwaves of a higher frequency range can be transmitted.
As is well known, the microstrip line can be used as a simple signal transmission line. In addition, if a suitable patterning is applied, the microstrip line can be used as microwave components including an inductor, a filter, a resonator, a delay line, etc. Accordingly, improvement of the microstrip line will lead to improvement of characteristics of the microwave component.
In addition, the oxide superconductor material (high T.sub.c copper oxide superconductor) which has been recently discovered makes it possible to realize the superconducting state by low cost liquid nitrogen cooling. Therefore, various microwave components using an oxide superconductor have been proposed.
However, it is almost impossible to assemble microwave equipment with microwave components using solely oxide superconductor based devices. Therefore, most microwave equipment using superconducting microwave components have both a low temperature part of its circuitry comprising the superconducting microwave components and a high temperature part comprising the conventional room temperature circuitry.
The resulting obstacles include a costly and complicated cooling system of the low temperature part and the difficulty of coupling of the low temperature part of circuitry with the conventional room temperature circuitry.